DIY_EFI Digest V4 #635

[Laura & Neil]

> Date: 11 Nov 99 00:32:16 +1200
> From: "Tom Parker" <parkert at ihug.co.nz>
> Subject: Re: Motronic 4.1 newbie--still struggling
>
> Derek Kaznoski <sweetpea1998 at hotmail.com> wrote:
>
> >I can hear a distinct hissing sound coming from around the bottom of the
> >plenum chamber where it intersects with the engine or just forward of this
> >area.  This hissing intensifies just after the throttle is blipped and
> >before the engine responds.  It continues with decreased intensity as the
> >rpm's rise.  To me, this sounds like an air leak, but I've never heard one.
> >Could this sound be coming from something else, such as circulating coolant?
>

This is something I have practical experience on.
Firstly, with the engine running use a rag and block the intake at the throttle
body. Do your best to stop any air entering the engine. If the engine still runs you
have a vacuum leak.
Now get a length of rubber tubing and carefully listen for the source of the leak.
If this doesn't work it's time for the carb cleaner.
Gently, and I mean really gently, spray a few drops of cleaner around any possible
leak sites. Not just the plenum but things like the brake power booster the evap
canister, just about anything that is vacuum operated. If the revs rise, you have
your leak. Disconnecting the O2 sesor helps, by not allowing the ECM to adjust the
mixture to compensate for the spray.
A few things to note
Carb spray burns real good, be careful.
Never assume there is only one leak.
The power steering pump can sound like a vacuum leak.

Neil